The antenna array aperture for mmWave is first introduced. It has been known that propagation of an electromagnetic wave is typically poorer at higher frequencies than lower frequencies. For example, the attenuation of electromagnetic waves around the millimeter wave (mmWave) frequency range would typically be higher than the attenuation around the micro wave frequency range as the path loss is usually more severe at higher frequencies. FIG. 1 illustrates such scenario with a transmitter (Tx) transmitting electromagnetic wave toward a receiver (Rx), and the mmWave aperture 101 is smaller than microwave aperture 102 since the mmWave has smaller wavelength. For an omni-directional antenna at both the transmitter and receiver, the received power (Pr) could be expressed as
                                          P            r                    =                                                    A                eff                            ⁢                                                P                  t                                                  4                  ⁢                  π                  ⁢                                                                          ⁢                                      d                    2                                                                        =                                                            (                                      λ                                          4                      ⁢                      π                      ⁢                                                                                          ⁢                      d                                                        )                                2                            ⁢                              P                t                                                    ,                              in            ⁢                                                  ⁢            which            ⁢                                                  ⁢                          A              eff                                =                                    λ              2                                      4              ⁢              π                                                          Equation        ⁢                                  ⁢                  (          1          )                    Aeff is the antenna effective aperture with λ being the wavelength, Pt is the transmit power and d the distance between the transmit antenna and receive antenna. The frequency of an electromagnetic wave has an inverse relationship from the wavelength of the electromagnetic wave, and the wavelength of the electromagnet wave is proportional to the size of the antenna. For example, if the propagating frequency is 30 GHz, then the wavelength is 10 mm; if the propagating frequency is 60 GHz, then the wavelength is 5 mm; and so forth.
Referring to FIG. 1, it can derived from Equation (1) that a system driving at a higher frequency would lead to a smaller antenna aperture and thus a lower received power. For example, an additional 20 dB extra loss could be incurred by the mmWave system as the propagating is increased from 3 GHz to 30 GHz. In such case, large arrays could be required in order to increase the aperture to compensate the path loss. On the other hand, additional losses such as the foliage loss which limits the coverage in forests and the heavy rains resulting in several dB losses in a 100 meter link may require larger margin in link budgets according to the high frequency operation.
Also due to higher propagating frequency, mmWave signals could be more sensitive to the blockages by some materials, such as metals or brick walls, in comparison to microwave signals. This would result in isolations of an indoor network from an outdoor network in the mmWave. Such phenomenon could be explained by comparing a line of sight (LOS) environment and a non-line-of-sight (NLOS) environment. FIG. 2 illustrates a LOS environment relative to a NLOS environment. Specifically, signals propagating in a LOS environment would be more like propagating in free space which exhibits a path loss exponent (PLE) in the range of 2˜3. However, signals propagating in the NOS environment would be much weaker, more sensitive to the environment, and exhibits the PLE in the range of 3˜4. For an accurate analysis without a loss of generality, incorporating the blockage effects in channel modeling could be needed.
In order to achieve a higher data rate, a larger bandwidth may be considered especially in mmWave wireless broadband systems. In such a system, the communication link with a larger bandwidth may lead to a higher noise power and thus a lower signal-to-noise ratio (SNR). FIG. 3 illustrates noise bandwidth in mmWave relative to noise bandwidth in microwave. As shown in FIG. 3, the mmWave noise bandwidth 301 would be greater than the microwave noise bandwidth 302. An extra noise power of 10 dB could be present from 50 MHz to 500 MHz. Therefore, a larger gain could be required in the mmWave communication system in order to compensate the power loss by using larger antenna arrays.
As for the beamforming operation for the mmWave communication system, there could be multiple beamforming schemes which includes digital baseband beamforming as shown in FIG. 4(a), analog baseband beamforming a shown in FIG. 4(b), and analog radio frequency (RF) beamforming as shown in FIG. 4(c). For power consumption and cost issues, the transmitter or the receiver may limit the number of RF chains. Thus, the analog RF beamforming as shown in FIG. 4(c) might be a good candidate in mmWave communications.
FIG. 5 illustrates examples of radiation patterns of different transmission wavelengths. In general, a communication system operating in the microwave band which has wavelengths in the centimeter range tends to have a small number of antennas. The radiation pattern of a single microwave frequency antenna 501 tending to be long distance, has a broad field-of-view (FoV) coverage, and is typical for a 3G/4G communication systems that use the micro-wave band with small number of BS antennas to achieve a higher receive SNR quality. However, low data rate due to small BS exists in such the systems. To increase the data rate by using a large BW, mmWave band is considered in the future communication system (e.g. 5G systems). The radiation pattern of a single mmWave single frequency antenna 502 covers a shorter distance; however, the mmWave radiation pattern 503 could be extended by using an mmWave antenna array for beamforming under the same transmitted power. Each of the BS beams 504 may have a different beam ID. In general, an mmWave communication system that uses a small sized antenna array tends to have a shorter distance and a broad coverage; whereas an mmWave communication system that uses a larger sized antenna array tend to have a longer distance and a narrower coverage.
The transmission framework of mmWave wireless communication systems could be classified into two categories based on the radio access interface. A first category is multiple radio access technology (multi-RAT) and a second category is single radio access technology (Single-RAT). FIG. 6 illustrates an example of a 5G multi-RAT communication system of the first category and the second category. The multi-RAT system has at least two RATs such as a LTE system and an mmWave system which have been phrased as the LTE+mmWave integrated system which would co-exist simultaneously for communications. For example, control signaling could be transmitted by using the conventional LTE communication frequency whereas the user data could be transmitted by using mmWave communication frequency. In such case, the carrier aggregation (CA) scheme could be utilized. The user data could be transmitted over the mmWave band by using, for example, a secondary component carrier (SCC), but control signals could be transmitted over the microwave frequency by using a primary component carrier (PCC). Network entry could be performed via the cmWave by using a PCC since a successful detection rate for control signaling could be operated in large coverage, high mobility and low SNR scenarios. On the other hand, the single-RAT communication system of the second category would use only one radio access technology for communication applications by using the mmWave band to transmit both user data and control signals. Network entry would be performed via a carrier in the mmWave band. Thus, a successful detection rate for control signaling may need to be operated in small coverage, low mobility and high SNR scenarios. To remedy this problem, beamforming technique may be used.
FIG. 7 illustrates a typical network entry procedure of a legacy communication system (e.g. 4G LTE system). After a UE powers on 701, the UE would perform at least a time/frequency synchronization (P-SCH & S-SCH) procedure 702, a cell-ID detection (S-SCH&RS) procedure 703, a (PBCH-MIB) broadcast signal detection procedure 704, a (PDDCH/SI-RNTI-SIB1) broadcast signal detection procedure 705, and a (PDDCH/SI-RNTI-SIB2) broadcast signal detection procedure 706. Subsequently, in step 711, the UE would initiate the random access procedure by transmitting a Random Access Preamble (RAP) to the serving BS (BS). The RAP could be used by the BS to calculating the timing advance (TA) parameter which is subsequently transmitted to the UE in step 712. In step 712, the BS would respond to the UE by transmitting a Random Access Response (RAR) back to the UE. The RAR would include various parameters such as TA, uplink (UL) grant, and TC-RNTI. In step 713, the UE would initiate network contention by transmitting to the BS a message that includes at least the S-TMSI which could be a 40 bit signal. In step 714, the BS would inform the contention resolution to the UE by transmitting a message which includes at least the C-RNTI to the UE. The C-RNTI could then be used by the UE to decode its data packets.
It should be noted that, as shown in FIG. 7, there is no beam related processing. For example, in the legacy LTE system, the reference signal transmitted from the BS to the UE as well as the subsequent signaling exchanges (e.g. 711˜714) would all use signals that are omni-directional. However, for communication systems that use beamforming for both control signaling and data packets by operating in the mmWave frequency, the current transmission scheme could be inadequate.